The ability of macrophages to release H2O2 has been correlated with their ability to kill bacteria and tumor cells in vitro. The mechanism by which the macrophage becomes activated to increase its production of H2O2, however, has not been clearly defined. A study has thus been made of a Iymphokine, termed H2O2 inducing factor (H2O2 IF), which is produced by human T cells. H2O2 IF has been found to stimulate human monocyte-like cells to increase their production of H2O2, which is measured by means of a colorimetric microassay based on the peroxide-mediated oxidation of phenol red. Using this assay, the H2O2 IF has been determined to have a mw of 54,000 and an isoelectric point of 5.5. In addition, the factor has been determined to have a buoyant density of 1.307 g/ml, indicating that the molecule is a protein.